Process for producing froth polyurethane foam

ABSTRACT

High quality, consistent polyurethane froth foams, both filled and unfilled, are prepared by directing polyol, isocyanate, and optionally other polyurethane-forming ingredients to a high pressure mix head prior to introducing the mixture to a froth foaming head. Changes in stoichiometry may be made rapidly without shut-down. An in-line blender incorporated filler into the polyol stream minimizing density differentials normally encountered in the holding tank, maintaining low and reproducible viscosity, and allowing for greater amounts of filler than otherwise possible.

This is a division of application Ser. No. 08/521,005 filed Aug. 30,1995 now allowed as U.S. Pat. No. 5,604,267, issued Feb. 18, 1997.

TECHNICAL FIELD

The present invention pertains to polyurethane froth foam. Moreparticularly, the present invention pertains to a process for preparingpolyurethane froth foam with enhanced processability.

BACKGROUND ART

Especially since the advent of the Montreal Protocol severely limitingthe use of CFC (chlorfluorocarbon) and other halogenated hydrocarbonblowing agents, frothed foams have become increasingly important. By theterm "frothed foam" and similar terms is meant a cellular foam productthe cells of which are formed by the mechanical incorporation of inertgas, particularly air, nitrogen, carbon dioxide, or argon, into a curingpolymer system, with or without the aid of small amounts of blowingagents of the physical or chemical types. Froth foams have been preparedfrom polymer systems such as SBR latex, PVC plastisol, and polyurethane,to the latter of which the present invention pertains.

Polyurethane froth foams have been used for numerous years, for examplein the preparation of foam-backed industrial carpet and carpet underlay.See, e.g., "Mechanically Frothed Urethane: A New Process for ControlledGauge, High Density Foam", L. Marlin et al., J. CELL PLAS., v. 11, No.6, November/December 1975, and U.S. Pat. Nos. 4,216,177; 4,336,089;4,483,894; 3,706,681; 3,755,212; 3,772,224; 3,821,130; and 3,862,879,which are herein incorporated by reference.

In the processes disclosed in these references, the polyurethanereactive components: the isocyanate ingredients (A-side), and polyolingredients (B-side) are each stored in separate, often heated, andsometimes aerated holding tanks. The two components are then meteredinto a standard froth foam mixing head at low pressure, to which is alsofed a supply of compressed air. The mix head contains mixing blades orsimilar devices moving at high speed, which whips air into the reactingmixture to produce a foam having a consistency not unlike shaving creamor whipped cream.

Due to the intensive mixing which occurs in the froth foam head, as wellas the air normally introduced either intentionally or unintentionallyinto the B-side holding tank, premixing of the reactive foam ingredientshas not been considered necessary. Mixers such as those from Hobart orOakes, of rather conventional construction, have been thought sufficientto provide thorough mixing of foam-forming ingredients. Even morethorough and efficient mixers include stator cylinders containingmultiple rows of pins within which revolves a rotor also carryingmultiple rows of pins which can rotate between the stator pins. Suchmixers are available, for example, from Lessco Corp., Dalton, Ga.

The froth foam is allowed to exit the mixer onto a conveyor belt onwhich, for example, a release sheet or carpet backing travels, isleveled with the aid of a roller or doctor blade, and generally passedthrough a curing oven or heated with radiant energy to cure the foam.For some uses, the foam is conveyed through a large diameter hose to thepoint of application. For many applications, for example carpetunderlay, considerable amounts of fillers such as calcium carbonate oralumina trihydrate are added to the B-side to increase the density andload bearing capacity of the foam.

Despite representing standard industry practice for many years, theprocesses previously described suffer from numerous drawbacks. Forexample, the change in ambient temperature which may occur betweenday-shifts and night-shifts or even between the morning and afternoon ofthe same shift can cause differences in the rate of the urethanepolymerization reaction. Changes in atmospheric moisture can also affectthe process as can changes in conveyor belt temperatures, etc., causedby continuous running of the process. In the past, changing processingchemistry past merely adjusting A-side/B-side ratios has requiredhalting the process, adjusting the A-side and/or B-side ingredients inthe holding tanks, and restarting the process. However, in most cases,the frothing head, and foam conveyor hoses when used, must be cleanedout. The result is loss of manufacturing time which increases cost ofthe product. In U.S. Pat. No. 4,925,508, for example, is proposed adisposable polyethylene or polystyrene pre-expansion chamber designed topartially reduce down-time.

In the manufacture of filled froth foam, further problems arise. Incommercial processes, fillers such as calcium carbonate or aluminatrihydrate are added to the B-side (polyol) in quantities up to 300parts per 100 parts polyol. The filler and polyol components areintensively high-shear mixed, and transferred to a holding tank which iseither unstirred or stirred with but modest agitation. Air may beincorporated into the filler/polyol to aid in the froth foaming processin addition to air supplied at the froth foam head, or may be"unintentionally incorporated" due to air entrained in the filler orincorporated from the head space above the polyol during high speedmixing. Once in the holding tank, however, entrained air tends to riseto the top while filler tends to settle to the bottom. There may be morethan a two-fold difference between the B-side density at the bottom ofthe tank and the top of the tank, i.e., 6 lbs/gal at the top and 14lbs/gal at the bottom. Since the pumps supplying the froth foam head arepositive displacement pumps, not only does the density of the productchange over time, but the polymerization chemistry changes as well dueto the variation in polyol content of the B-side caused by movement ofair and filler.

To counteract the difference in density, some processes link the lowpressure positive displacement pumps with mass flow devices whichmeasure mass flow rather than volume flow and adjust volume flowaccordingly in a closed loop process. While such measures maintaindensity, they do not maintain chemical stoichiometry, but rather canadversely affect stoichiometry, since the less dense B-side, the volumeof which the closed loop process will cause to increase, may alreadycontain a higher weight percent polyol than that desired.

Also important in filled systems is the phenomenon of B-side viscosityincrease over time. Over time, the filler/polyol mixture increasesconsiderably in viscosity, perhaps due to greater wetting of the fillersurfaces with polyol. It is not uncommon for the viscosity to increasefrom 2000 cps to 4000 cps over a time of two hours, for example. Theincreased viscosity reduces pumping efficiency, and more importantly,adversely affects the frothing operation. The result of this and theforegoing factors make continuous production problematic. It is notuncommon for production to be halted every few hours to adjust processparameters, with the deleterious effects on process time previouslydescribed.

At times, it is desirous to provide a froth foam product which ismultilayered, for example a first layer of lower density and higherresiliency and a second layer of higher density and lesser resiliency.In the past, production of such products has met with but limitedsuccess. At the exterior of the first produced foam surface, the frothexhibits coalescence, forming relatively large cells. Since the secondfroth foam layer, like the first, does not exhibit the expansion typicalof blown polyurethane foams which might be sufficient to force theexpanding polyurethane into the surface of the first layer, a secondfroth foam layer does not adhere well to the first layer, resulting inthe potential for delamination during production and/or use.

It would be desirable to provide a process for the production ofpolyurethane froth foam in which the polyurethane stoichiometry can beadjusted on the fly, rather than requiring shut down. It would befurther desirable to provide a process for the preparation of filledpolyurethane froth foam in a consistent and reliable manner withoutresorting to use of mass flow meters and other devices. It would be yetfurther desirable to provide a process for producing polyurethane frothfoam wherein multiple layers of foam may be successfully applied. It isfurther desirable to provide a process where uniform froth foam can beproduced, even at low density.

SUMMARY OF THE INVENTION

It has now been surprisingly discovered that consistent, high qualitypolyurethane froth foam may be produced in a process where thepolyurethane reactive ingredients are first delivered to a standard highpressure mix head prior to entry into a standard froth foam head. In apreferred embodiment, filled froth foams are produced by blending fillerin-line with the polyol stream prior to entry into the high pressure mixhead. The froth foam produced exhibits more uniform cell structure thanprior art froth foams, and unexpectedly generates a smooth interfacewith minimum coalescence, allowing for production of high qualitymultiple layer froth foam products.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a schematic of one embodiment of the process of thesubject invention;

FIG. 2 is a scanning electron photomicrograph of a froth foam of thesubject invention;

FIG. 3 is a scanning electron photomicrograph of a two layered frothfoam of the subject invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The process of the subject invention may be described with reference tothe process schematic of FIG. 1. In FIG. 1, the polyol B-side iscontained in holding tank 101. The B-side may be all polyol, or polyolwith chain extenders, cross-linkers, catalysts, and other additives andauxiliaries known to the field of polyurethanes. The polyol is pumpedfrom holding tank 101 through optional in-line blender 103 by suitablemeans, for example variable speed metering pump 105. High pressure pump107 injects the B-side into high pressure mix head 109, whichadvantageously may be of the impingement mixing type.

At 111 is the A-side isocyanate tank, from which isocaynate is directedto the high pressure mix head 109 by high pressure pump 113. Compressedair is directed to the high pressure mix head through line 115, theamount of air determined by the air flow meter 117, and the volumecontrolled by valve 119 or by adjusting the pressure of supply air. Fromthe high pressure mix head 109, the reactive mixture flows to froth foammixing head 121 which may also include an inlet 122 for additionalfrothing gas, from which it is dispersed onto a conveyor or through ahose to the point of application.

The optional blender 103 consists of a standard liquid/solid blenderhaving a supply hopper 123 containing filler, which is metered into theblender by means of auger 125. Standard techniques are used to measureand adjust filler weight added to the polyol.

Also shown in FIG. 1, entering high pressure mix head 109 are inlets 127and 129 which may be used to supply additional streams of polyurethaneingredients such as catalyst solutions, cross-linkers, surfactants,colorants, additional isocyanate or polyol, auxiliary blowing agents,e.g. water, low boiling hydrocarbons, CFC-22, and the like. Preferably,the process is performed without auxiliary blowing agents. Additionalinlets to the mix head may be provided as well, the various inletsproviding for maximum flexibility in polyurethane stoichiometry.

The various components are standard components and readily available.Variable speed low and high pressure pumps are standard, off-the-shelfitems available from numerous suppliers. A suitable filler/polyolblending unit is a Turburlizer I filler blender, available from DarwinEnterprises, Inc., Dalton, Ga. Other blenders are suitable as well.Likewise, high pressure mix heads are available from sources such asCincinnati Milicron, Elastogran GmbH, Hennecke, and other suppliers. Asuitable high pressure mix head is a carpet backing foam machine headavailable from Hennecke Equipment Co., Pittsburgh, Pa. Suitable frothfoam heads include those available from Hobart, Oakes, and Lessco. Apreferred froth, foam head is a "Firestone" type head designated LesscoSystem Superfoam Blender available from Lessco Corp., Dalton, Ga.

Suitable formulations for preparing froth foam are disclosed in thenumerous references cited earlier, and are well known to those skilledin the art. A preferred froth foam formulation is ARCOL® froth foam mixavailable from ARCO Chemical Co., Newtown Square, Pa., which employs, inaddition to polyol, silicone surfactant L5614 and urethane-promotingcatalyst LC-5615, both available from OSi, Inc., and isocyanate E-448available from Bayer, Pittsburgh, Pa.

The polyurethane froth foam formulation itself forms no part of thepresent invention, and many formulations are suitable. The filler may beany filler generally used, e.g. calcium carbonate, alumina trihydrate,talc, various clay minerals, e.g. bentonite, or mixtures of these.

FIG. 2 is a scanning electron photomicrograph of a section taken througha foam of the present invention. Noteworthy is the uniformity of thecell structure and the presence of large numbers of complete cellsdespite the shear required to provide an edge suitable for examination.FIG. 3 is a similar photomicrograph of a two layer froth foam product.Noteworthy is the fine and uniform interface between the two layers, thefirst layer showing virtually no coalescence. The two layer foam wasproduced by curing the first layer prior to application of the secondlayer (wet on dry).

The advantages of the subject process are numerous. In addition toproviding a high quality product, even in the lower density ranges, twolayer or multiple layer quality foams may be produced. Moreover, thestoichiometry of the product may be readily adjusted, either manually orunder computer control, by adjusting the volume of the various feedstreams to the high pressure mix head. The process is particularlyflexible when rather than merely A- and B-side streams, individualcomponents are supplied to the mix head.

Of particular note, however, is the uniformity produced in filled frothfoams when filler is added to polyol in the in-line blender. Since thefiller/polyol blend is injected into the high pressure mix head afteronly a short time, the viscosity of the blend remains low and exhibitslittle or no variation in viscosity. Moreover, due to the absence ofincreased viscosity over time, larger amounts of filler can be used,which otherwise, in a conventional process, would render the B-side tooviscous or even gelled or thixotropic. Suitable amounts of filler rangeup to 450 parts per 100 parts by weight polyol, preferably 50 parts to450 parts filler per 100 parts polyol.

Most especially advantageous in filled foams is the lack of densityvariation of the B-side/filler blend seen when conventional holdingtanks are used. The stoichiometry and density both remain essentiallyconstant over extended periods of time, and the process can be conductedwithout the complication and added expense of mass flow meters andassociated equipment.

Having now fully described the invention, it will be apparent to one ofordinary skill in the art that many changes and modifications can bemade thereto without departing from the spirit or scope of the inventionas set forth herein.

What is claimed is:
 1. A multilayer froth foam product comprising two ormore individual layers of flexible polyurethane froth foam, wherein aninterface between at least two of said two or more individual layers ofpolyurethane froth foam is substantially free of coalescence.
 2. Themultilayer froth foam product of claim 1 wherein at least one layer ofsaid two or more individual layers of polyurethane froth foam contains afiller in an amount of from 50 parts to 450 parts filler per 100 partspolyol in a polyol component used to prepare said at least one layer. 3.The multilayer froth foam of claim 1 wherein said individualpolyurethane foam layers are each individually prepared by the processof delivering polyurethane reactive components comprising an isocyanatecomponent and a polyol component to a high pressure mix head; deliveringcompressed gas said high pressure mix head to form a frothablepolyurethane reactive mixture; frothing said frothable polyurethanereactive mixture to form a curable polyurethane froth foam; forming acurable polyurethane froth foam layer from said curable polyurethanefroth foam; and curing said curable polyurethane froth foam.
 4. Themultilayer froth foam of claim 3 wherein said polyol component containsa filler in the amount of from about 50 parts to 450 parts by weightfiller per 100 parts polyol.
 5. A multi-layer froth foam productcomprising two or more individual layers of polyurethane froth foam,wherein an interface between at least two of said two or more individuallayers of polyurethane froth foam is substantially free of coalescence,second and subsequent layers of said two or more layers of polyurethanefoams having been prepared by contacting a reactive polyurethane frothdirectly with a prepared foam layer such that the two layers are inimmediate contiguous contact.